Many aftermarket automotive telematics devices plug into a vehicle's onboard diagnostics (OBD) port, because this port is quite standard across a large variety of vehicle make/model/year. The OBD port conveniently provides power and ground connections that are used to power the telematics device. In some cases, the telematics device may be plugged into the front side of the OBD port, i.e. into the connector socket that faces into the passenger compartment. In this case, it is easy and convenient for an end user to install or un-install the telematics device.
In other cases, the aftermarket telematics device may plug into the backside of the OBD port, which may be behind or under the dashboard. This can be done most conveniently with a T-harness. As delivered from the factory, vehicles typically have a cable with a standard OBD connector that connects to the vehicle OBD port. If this cable is unplugged, the T-harness can then be inserted between this cable and the OBD port, and the telematics device can be connected to the secondary leg of the T-harness. This type of installation is very convenient because it is standard across a large variety of vehicle make/model/year and does not require additional wiring or wire splicing to any other part of the vehicle. This type of installation can also mitigate the possibility of a vehicle warranty violation.
Some telematics devices that monitor functions of heavy-duty vehicles are designed to receive power from a J-Bus port. The J-Bus serves generally the same purpose in heavy-duty vehicles as does the OBD port in passenger vehicles.
While the OBD and J-Bus ports conveniently provide power to the aftermarket telematics device, this power is “always-on” (supplied by the vehicle battery), even when the vehicle ignition is OFF. As such, the telematics device should be designed to minimize the power usage so as not to excessively drain the vehicle battery when the ignition is OFF. This is typically done by including enough intelligence in the telematics device to operate in a special “low-power” mode, or even turn off completely, when the ignition is OFF. However, the OBD and J-Bus ports do not provide a standard power status signal or switched-power from which the telematics device can ascertain whether the ignition is ON or OFF.
Various inconvenient or unreliable methods have been used in prior aftermarket telematics devices to enable and disable low-power modes. One such method is to wire an actual signal from elsewhere in the vehicle that indicates the ignition status of the vehicle. This defeats the convenience of the OBD and J-Bus port installation as it requires separate wiring (and possibly splicing) that is different on each vehicle. Another possible method is to monitor the voltage on the OBD or J-Bus power port, as often this voltage will be higher than the battery voltage when the vehicle's alternator is running. This method is unreliable.
What is needed, therefore, is a reliable technique for using one or more signals in the OBD or J-Bus port of a vehicle to determine the ON/OFF status of the vehicle.